Another question is: When did the decline of Trek begin? I'd argue for the case that it waned after DS9 ended. VOY couldn't hold its own without DS9 there to support it. And Insurrection, Nemesis, and ENT all failed.
I agree with this, but I think there's also a systemic element: VOY, regardless of its flaws or merits as entertainment, also reflects the shift from syndication-where Trek was offered directly by the studio and something of a heavy hitter-to the rise of the mini-nets, where Trek was subject to a supernumerary layer of executive oversight and confined to a niche. UPN was never a terribly competently-managed network, and Trek as a property was at the mercy of whatever narrow demographic strategy was in vogue at the network at a given time.